The invention relates to a piston ring forming method and apparatus, and more particularly to a method and apparatus for inducing a slight additional inward curvature, often called negative point protrusion, on either side of the split of a piston ring.
The split piston rings formed by conventional processes are generally somewhat out of round in order that the ring exert an even pressure around the cylinder wall when it is compressed and installed therein. Sometimes the ends of such a ring tend to spring outwardly to a greater extent than the rest of the ring due to traditional manufacturing techniques. This tendency can cause problems in internal combustion engines having gas ports within the cylinder walls. The ends of a ring may spring slightly outwardly as they pass over a valve port opening, then snag upon the upper or lower edge of the gas port as the piston continues to travel. Such "port clipping" causes scuffing and scoring of both the ring and cylinder, which can cause senious difficulties and result in early wear. It is therefore desirable to produce a slight additional inward curvature on the piston ring at each end adjacent the split. This negative point protrusion of the ring brings the ring's ends slightly inward within the cylinder so that they cannot catch on the edges of the gas ports.
Previously, face lapping of a ring, wherein the ring is compressed into a cylinder similar to a firing cylinder and reciprocated therein with a fine grinding compound, was sufficient to prevent serious problems of ring end snagging in service as described above. The stresses within the ring would be more or less equalized during lapping by removal of material from areas which tend to exert greater outward pressure, including the ring ends. However, certain newer engines have placed piston rings under more demanding conditions, often requiring actual recession of the ring ends to avoid snagging on ported valves. Particularly in turbocharged engines, which involve much higher pressures within the firing cylinders, pressurized gases entering the space between the piston and the ring exert greater outward pressures on the ring causing it to push outwardly on the cylinder wall with greater force. As a result, the ring ends are forced outwardly enough to allow them to catch on the ported valves of the cylinder wall.
Special grinding operations such as "cam" profile grinding have been used to remove material from the outer faces of ring ends, resulting in a recession of the ring face at its ends. Also, negative point protrusion of piston ring ends has been done by cold bending. However, these methods can result in irregularities in surface finish, uneven bending, and some loss of ring strength. The methods are also generally more costly than that of the present disclosure.
Heat treatments have been used in a variety of ways in the manufacture of piston rings. See, for example, U.S. Pat. Nos. 2,487,587, 2,280,552 and 3,377,682. In U.S. Pat. No. 2,487,587, ring edges are hardened through induction heating. Pat. No. 2,280,552, also relating to ring hardening by heat treating, recognizes that a split ring expands as a result of localized heat treatment. In U.S. Pat. No. 3,377,682,a coil of rings is radially stretched, then heat treated to "set" the stretch. In U.S. Pat. No. 2,081,257, packing rings are deformed into a smaller radius under pressure, then heated to keep them in the deformed configuration.